This invention is in the field of finishing processes for melt-fabricable fluoropolymer resins, particularly for making beads from aqueous dispersions of such fluoropolymers.
Melt-fabricable fluoropolymers are well known. Such polymers include vinylidene fluoride (VF2) homopolymers and copolymers, copolymers of ethylene with tetrafluoroethylene (TFE) or chlorotrifluoroethylene (CTFE), and copolymers of TFE with hexafluoropropylene (HFP) and/or perfluoro(alkyl vinyl ether) (PAVE). They are considered to be melt-fabricable because they can be fabricated into useful articles by conventional methods such as melt extrusion and injection molding. Since a large fraction of melt-fabricable fluoropolymers is processed by such techniques, such fluoropolymers are most commonly sold as cubes suitable for feeding to extruders and injection molding machines. Cubes, typically having dimensions of the order of 3 mm, are usually prepared by melt extruding raw polymer into coarse strands and then cutting the strands.
Such cubes are not suitable for all uses, and other physical forms of melt-fabricable fluoropolymer such as powders and aqueous dispersions are also supplied. One use for which extrusion cubes are not suitable is rotational casting (rotocasting), which encompasses rotational molding (rotomolding) and rotational lining (rotolining). Rotocasting requires a combination of good solid-state flow of the fluoropolymer particles to facilitate uniform distribution of resin in the final article, and a spherical shape and small particle size, in the range of 0.1-0.5 mm, to promote a smooth profile on the free surface of the article. The free surface is that surface not in contact with the surface of the mold or with the surface of the article to be lined. A convenient product form for this purpose is a bead or granule such as that disclosed by Buckmaster et al. in Canadian Patent 1,248,292. Such beads can be prepared, as therein disclosed, by agitation of aqueous dispersion of fluoropolymer particles in the presence of electrolyte and water-immiscible organic liquid, a process known as solvent-aided coagulation or as solvent-aided pelletization (SAP). Optionally, after isolation from the liquid, the resultant beads can be subjected to treatments to harden them and/or to stabilize any unstable end groups present on the fluoropolymer.
Water-immiscible organic liquids that can be used in SAP processes include the aliphatic hydrocarbons such as hexane, heptane, gasoline and kerosene, aromatic hydrocarbons such as benzene, toluene and xylene, halogenated derivatives such as carbon tetrachloride, monochlorobenzene, the trichlorotrifluoroethanes, difluorotetrachloroethanes, and liquid oligomers of chlorotrifluoroethylene as disclosed in Canadian Patent 1248292, in which 1,1,2-trichloro-1,2,2-trifluoroethane (CFC-113) is used in the examples. Hydrocarbons are hazardous because of their flammability, while chlorocarbons and CFCs present environmental problems. Alternative water-immiscible liquids for use in SAP processes include the hydrofluorocarbons (HFC) disclosed by Takakura and Funagi in Japanese Patent Application Publication (Kokai) H07-2783 14. While such HFCs have low ozone depletion potential (ODP), they do have global warming potential (GWP). For example, one isomer of C5H2F10(HFC 43-10) has GWP of about 1300. Furthermore, HFCs can be sensitive to alkaline gelation agents, which restricts their application to acidic gelation processes. Additional environmentally friendly solvents are needed for SAP.
The SAP process is complicated in the sense that many variables are involved. Any change, e.g., a new solvent, may require adjustment of other variables. A process for preparing beads that has a broader operating window, i.e., is less sensitive to the variables, is desired.
This invention provides a process comprising forming beads of melt-fabricable fluoropolymer resin by solvent-aided pelletization of aqueous dispersion of particles of said resin, wherein said solvent-aided pelletization is carried out using fluorinated solvent containing oxygen in which the oxygen is present only as ether oxygen. Fluorinated solvents containing ether oxygen include (a) perfluorinated cyclic amines of formula (I) hereafter, (b) hydrofluoroethers having the formula Rxe2x80x94Oxe2x80x94Rf, wherein R is alkyl having 1-3 carbon atoms and Rf is linear or branched fluoroalkyl having 2-7 carbon atoms and containing no halogen other than fluorine and at most one terminal hydrogen atom, and (c) hydrofluoroether having the formula R4xe2x80x94Oxe2x80x94R5, wherein R4 is a fluoroalkyl having 1-6 carbon atoms and at least one hydrogen atom and R5 is linear or branched fluoroalkyl containing no halogen other than fluorine and having 1-7 carbon atoms and optionally containing ether oxygen. Preferred solvents include hydrofluoroethers.
The process of the invention has environmental advantages, and also yields small well-formed beads over a broad range of solvent-to-polymer ratio, extending to low solvent-to-polymer ratios.
It has been discovered that organic liquids having oxygen present only as ether oxygen can be used in SAP processes for agglomeration of melt-fabricable fluoropolymer particles in aqueous dispersion. Preferably, the organic liquids have one ether oxygen per molecule.
Organic liquids that can be used to form beads of melt-fabricable fluoropolymer resin by the SAP process of the present invention include perfluorinated cyclic amines having a nitrogen atom in the ring and having an ether oxygen in the ring. These compounds have no ODP.
Among the perfluorinated cyclic amines useful in this invention are those having the general formula 
in which R1 is a linear or branched saturated perfluorocarbon group having 1-4 carbon atoms, R2 and R3 are linear or branched saturated perfluorocarbon groups having, independently, 1-5 carbon atoms, and the total number of carbon atoms in the molecule of formula (I) is 3-10.
Examples of compounds of formula (I) include perfluoro-N-methylmorpholine (PFNMM) and perfluoro-N-isopropylmorpholine. Preferred compounds of formula (I) are those in which the total number of carbon atoms is 4-8 and the number of atoms bonded in the ring is 5-6. Especially preferred compounds are those in which R2 and R3 are xe2x80x94CF2CF2xe2x80x94, and R1 has 1-3 carbon atoms. PFNMM and perfluoro-N-isopropylmorpholine are available commercially (Fluorinert(copyright) PF-5052 and FC-6003, respectively, 3M Company). PFNMM is most preferred.
Organic liquids that can be used also include hydrofluoroethers (HFE) having the formula Rxe2x80x94Oxe2x80x94Rf, wherein R is alkyl having 1-3 carbon atoms, preferably 1-2 carbon atoms, and Rf is linear or branched fluoroalkyl, preferably linear, containing no halogen other than fluorine and having 2-7 carbon atoms, preferably 4-5 carbon atoms, and at most one terminal hydrogen atom. Preferably, Rf is perfluorinated. Examples of HFE that can be used include perfluorobutyl methyl ether (CH3xe2x80x94Oxe2x80x94C4F9) and perfluorobutyl ethyl ether (C2H5xe2x80x94Oxe2x80x94C4F9). These are commercially available as HFE-7100 and HFE-7200 respectively, from 3M Company, St. Paul Minn. USA. Other suitable HFE have the formula R4xe2x80x94Oxe2x80x94R5, wherein R4 is a fluoroalkyl having 1-6 carbon atoms, preferably 1-3 carbon atoms, and at least one hydrogen atom, preferably on the carbon adjacent to the oxygen atom, and R5 is linear or branched fluoroalkyl, preferably linear, containing no halogen other than fluorine and having 1-7 carbon atoms, preferably 2-4 carbon atoms and optionally containing ether oxygen. Preferably R5 is perfluorinated. Examples include CF3xe2x80x94CHFxe2x80x94Oxe2x80x94CF2xe2x80x94CF2xe2x80x94CF3 and CF3xe2x80x94CHFxe2x80x94Oxe2x80x94CF2xe2x80x94CF(CF3)xe2x80x94Oxe2x80x94CF2xe2x80x94CF2xe2x80x94CF3, the preparation of which is described in Preparation, properties, and industrial applications of organofluorine compounds, by R. E. Banks, John Wiley and Sons, NY, 1982, page 100.
Among organic liquids having oxygen present only as ether oxygen, the HFE described above are preferred.
The process of this invention may employ any of the techniques known to use a water-immiscible organic liquid (solvent) to pelletize particles of fluoropolymer initially present in an aqueous medium. Typically, a vessel equipped with baffles is charged with a quantity of aqueous dispersion of fluoropolymer particles. The aqueous dispersion is agitated with an impeller rotating at a chosen speed, and, with agitation continuing, an electrolyte (sometimes called a gelation agent) is added. A quantity of solvent is added, either simultaneously with electrolyte addition or sequentially, and agitation is continued for several minutes, optionally at different speed. The resultant product is filtered to drain away the liquid, and the beads are dried, usually at elevated temperature and optionally under vacuum. One skilled in the art will recognize that many variations of this process are possible within the scope of the present 25 invention. For example, the temperature of the mixture can be increased toward the end of the process to boil off the solvent before screening the beads from the liquid. This alternative can facilitate solvent recovery.
Optionally, the resultant beads can be subjected to various treatments. For example, the beads can be heat hardened by baking them for several hours, e.g., 3-24 hr, either under nitrogen or in air, at a temperature near but below the melting point of the fluoropolymer as determined by differential scanning calorimetry (DSC), e.g., within 30xc2x0 C. of the melting point. Drying and heat hardening can be combined if drying is carried out at high temperature. Additionally or alternatively, for example, the beads can be treated with elemental fluorine to reduce the concentration of unstable end groups as disclosed by Canadian Patent 1248292 or by Imbalzano and Kerbow in U.S. Pat. No. 4,743,658.
Preferably, the fluoropolymer particles are produced by dispersion polymerization as known in the art. The dispersion thus produced is referred to as xe2x80x9craw dispersionxe2x80x9d. For solvent assisted pelletization, the raw dispersion may be used directly or after addition of water to adjust the solids concentration. Average fluoropolymer raw dispersion particle size (RDPS) is generally in the range of 50-350 nm, preferably in the range of 100-300 nm, more preferably in the range of 150-250 nm. The concentration of polymer solids in water is usually in the range of about 10-30 wt %, preferably 14-25 wt %, based on combined weight of polymer particles and water. While temperature is a variable in pelletization, the temperature at which the agglomeration bead-forming process is carried out can be any convenient temperature between the freezing point of water and the boiling point of the solvent used. Room temperature is convenient and preferred for this reason, but higher temperature favors formation of smaller beads. The amount of electrolyte used will vary with the strength of the electrolyte, and is sufficient to cause formation of a gel, hence the term xe2x80x9cgelation agentxe2x80x9d. Generally, the amount of electrolyte is in the range of about 0.4-10 wt %, preferably 0.5-5 wt %, based on the dry weight of polymer solids. The amount of solvent used is generally such that the solvent-to-polymer (S/P) ratio on a weight basis is in the range of about 0.4-1.4, preferably 0.5-1.0. The duration of agitation before electrolyte addition, the duration of agitation after gelation, and the duration of agitation after addition of the solvent can be selected independently. For efficiency reasons, it is desirable that the process be as short as possible. The three recited times are usually of the order of minutes. However, the agitation profile is usually chosen to ensure that the structure of the gel is maintained, that is, that the gel is not broken into separate phases before the solvent is added. Thus, the agitation time is relatively short and the agitation speed is relatively low in the intervals before solvent addition. The absolute agitation speed used, of course, will vary with the scale of the equipment. Typically, the duration of agitation after solvent addition is relatively long to enhance the uniformity of the resultant beads. Agitation speed after solvent addition influences the size of the beads, and may be faster than in earlier intervals, faster agitation generally resulting in smaller beads. Typically, the preliminary agitation period is in the range of about 0-5 min, the agitation period after electrolyte addition is in the range of about 0-20 min, and the agitation period after solvent addition is in the range of about 1-10 min.
Melt-fabricable fluoropolymers that can be used in the process of the present invention are made from at least one fluorine-containing monomer, but may incorporate monomer(s) which contain no fluorine or other halogen, and preferably contain at least 35 wt % fluorine. Examples of melt-fabricable fluoropolymers include copolymers of TFE with one or more copolymerizable comonomers chosen from perfluoroolefins having 3-8 carbon atoms and perfluoro(alkyl vinyl ethers) (PAVE) in which the linear or branched alkyl group contains 1-5 carbon atoms, with comonomer(s) in the copolymer present in amount(s) sufficient to reduce the melting point of the copolymer substantially below that of TFE homopolymer, e.g., to a melting point no greater than 315xc2x0 C. Preferred perfluoropolymers include copolymers of TFE with at least one of hexafluoropropylene (HFP) and PAVE. TFE/PAVE copolymers are especially preferred. Preferred comonomers include PAVE in which the alkyl group contains 1-3 carbon atoms, especially 2-3 carbon atoms, i.e., perfluoro(ethyl vinyl ether) (PEVE) and perfluoro(propyl vinyl ether) (PPVE). Additional fluoropolymers that can be used include copolymers of ethylene with TFE (ETFE), optionally including minor amounts of one or more modifying comonomer such as perfluorobutyl ethylene (PFBE). Other hydrogen-containing fluoropolymers that can be used include copolymers of ethylene and chlorotrifluoroethylene (ECTFE), and vinylidene fluoride homopolymers and copolymers. Generally, melt-fabricable resins have melt viscosity (MV) in the range of 0.5-50xc3x97103 Paxc2x7s though viscosities outside this range can be used. MV is measured according to ASTM D-1238 at the temperature appropriate for the particular fluoropolymer. Preferably, MV is in the range of 1-35xc3x97103 Paxc2x7s. Such fluoropolymers can be produced by aqueous dispersion polymerization as known in the art.
Beads produced by the process of the present invention can have a wide range of dimensions, such as D50 of 100-3000 xcexcm measured as described below. D50 of 200-1000 xcexcm is preferred.